The use of organic thin film deposition in commercial products has grown considerably over the last decade. In particular, the use of organic thin film structures in small, precise, high aperture ratio applications such as OLED displays has been of increasing commercial interest. The OLED deposition process is one example of numerous organic thin film applications and provides a good example of the requirements and limitations of conventional organic thin film disposition structures.
Conventional OLED display structures are built on a substrate in a manner such that a two-dimensional OLED pixel array is formed. Each OLED in the array includes overlying layers starting with an electrode formed on the substrate, followed by an organic electroluminescent (EL) emission medium deposited over the first electrode and an electrode on top of the EL emission layer. At least one of the electrodes is transparent. The size and shape of the organic EL medium deposit is controlled through the use of shadowmasks, which are generally metal plates with patterned apertures that cover the substrate during the deposition process. A source of organic material deposits the EL material through the aperture in the shadowmask upon the glass substrate in a pre-designed pattern of pixels. Full color displays typically comprise of red, green, and blue subpixels. For full color OLED displays some layers may be common to all subpixels (such as the hole and electron transport layers) while others may be unique to a specific color subpixel (such as the EL emission layer). For common layers, a shadowmask is often not used; if it is used, the shadowmask aperture is usually large and typically does not require highly precise tolerances. Unique layers, however, are typically deposited on individual subpixels, which are very small, and typically require highly precise tolerances to avoid deposition in adjacent subpixels of different colors. Once the deposition of EL medium is complete, the shadowmask is taken off the substrate. There are currently two primary shadowmask manufacturing techniques for producing the precision shadowmasks required for patterning fine resolution depositions: electroless plating and etching.
One specific challenge in the manufacture of OLED displays is finding a method to minimize the distance between pixels in order to maximize the quality of resolution in an OLED display as well as to enhance the flexibility of manufacturing varying sizes of OLED displays. The precision shadowmask apertures formed by electroless plating have sharp edges and corners, ideal for matching the rectangular shaped pixels of a display. These sharp edges, however, create a severe “shadowing effect” due to the varying angle of deposition, from a point source or as the linear source travels across the shadowmask, during EL medium deposition. This shadowing effect causes the thickness of the typically rectangular shaped deposition to reduce towards the edges. In contrast, the precision shadowmask apertures formed by etching have tapered edges that help minimize the shadowing effect, but the etching process also produces apertures with rounded corners that result in a deposition structure with rounded corners. When utilizing either an electroless plated or etched precision shadowmask, organic material must overlap the pixel area by an average of 15 microns in order to ensure complete pixel fill and uniform thickness in the emitting region. This overlap limits how closely pixels can be spaced together and therefore negatively effects aperture ratio.
One system of transferring organic medium upon a substrate is described in U.S. Pat. No. 5,844,363, which describes an OLED in which the hole-transporting layer, the electron transporting layer and the emissive layer, are comprised of a non-polymeric material. A method for preparing OLEDs using vacuum deposition techniques and a shadowmask are further disclosed. While U.S. Pat. No. 5,844,363 describes an improved OLED, it does not address improving OLED pixel resolution or aperture ratio, improvements in the method of manufacture of thin film structures, or improvements in either electroless plated shadowmasks or etched precision shadowmasks.